You forget AMD. Their SMU firmware in northbridges is signed (I don't know if there are public results on how effective it is or how optional the firmware is), it does some tasks related to power management: like what the NVIDIA features restricted to signed firmware do. (AMD has or will have something like AMT, possibly with similar problems for free systems.)
I think the AMD GPU microcode is not signed, replacing it might be the best way on x86 systems (as hard as what Nouveau currently does, but with publicly redistributable blobs: less motivation for distro vendors to support reverse engineering). Or NVIDIA won't decide to make necessary features require signed firmware.
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